Oct 21, 2011, 10:07 PM  
United States, TX, Huntsville
Joined Jun 2011
142 Posts

Discussion
how to compare electric motors
I am used to nitro motor sizes which was fairly easy to learn but all the numbers with brushless motors are beating me up. such as the kv how many poles the ######## stats and some motor pages dont even mention prop sizes, So what is the easiest way to compare them? for example
http://www.hobbypartz.com/96m601big...201100kv.html to http://www.horizonhobby.com/Products...32A#quickSpecs 

Oct 22, 2011, 12:37 AM  

KV is RPM per Volt. Its useful when comparing motors of similar size, which the two you linked are not.
A high KV is good for a fast prop speed, gear box set up, or when using a low voltage battery. Low KV motors will swing larger props at slower speeds, but you can generally get more static thrust out of them. There are several variables at play. What kind of performance do you want, what prop, what battery. Once again, to easily compare KV you need similar motors. The Tacon Big Foot 10 and the Power 32 are apples and oranges. It may be more useful to provide the situation where the motors will be used. Then we can give more specific comparisons. Edit: to be clear, power (watts) has the most direct affect on how your plane will fly 
Oct 22, 2011, 01:29 AM  
United States, WA, Pasco
Joined Aug 2011
32 Posts

I've got a question:
I purchased this motor: http://www.hobbyking.com/hobbyking/s...dProduct=18157 Specs. Turns: 8T Voltage: 2~3S Lipoly RPM/V: 1500kv Internal resistance: 0.026 Ohm Max Loading: 31A Max Power: 370W Shaft Dia: 3.175mm Weight: 80g It says 23S, and at 3S I measured 30amps with a 7.3 inch prop, and it spiked up to 70A with an 11.3 prop. What can I expect if I ran it with a 2S? Will I be able to use an 11.3 prop at lower kv if I use a 2S and stay near or under 30A? I'd like to be able to use this for a foamy 3D plane. 
Oct 22, 2011, 02:21 AM  

You can use any of the many electric motor calculators to give you a decent guess at what you can expect for specific set ups. There are also prop test charts out there as well. They can give you a good idea about real world setups.
All that information can be found in the power systems forums. Dropping down to a 2S battery will let you swing a larger prop, but you would be hard pressed to match the power you would get from the 3S swinging a smaller prop. For a 3D plane, I would stick with the 3S. I literally have a box with about 40 different prop brands and sizes. I'll use prop charts or calculator programs to get me close, then do my own testing to find the right prop. Airplanes will fly on about 50 watts/lb Sport flying is around 120 watts/lb Hovering is about 150160 watts/lb Most recommended 3D set ups will be around 200 watts/lb 
Oct 22, 2011, 06:06 AM  

Those two motors are not comparable in any way. The Tacon is the rough equivalent of a .10 size glow engine and the EFlite 32 is equivalent to a .32. KV is a measure of maximum motor RPM (unloaded). Multiply the KV by the voltage to get RPM. The best way to do this is to start with the model you want to power and the flying style you want for it. Bring that to the forum and you can get some more useful recommendations.

Oct 22, 2011, 09:09 AM  
Joined Jan 2009
7,643 Posts

Quote:
But that's a bit sloppy. More commonly we go by watts per pound of flying weight (aka allup weight.) For a mediumsize sport plane like the Pulse 25, 100 watts/pound is "adequate" and 125150 watts/pound starts to get exciting. The Pulse 25 is around 4 lbs. AUW so you need... let's call it 500600 watts. The first EFlite Power 25 motor I found via google says it weighs 190 grams... by the threewattpergram rule, that would be 190*3 watts = 570 watts. Going by the motor's specs (for the 870 Kv version): Max cells given as 34S, max current 32 amps. 4 Lipo cells is 4*3.7 volts 14.8 volts. 14.8 volts times 32 amps is 473 watts. For the Power 32 motor: 42 Amps max, 35 Lipo cells. That works out to 3.7*5*42 = 777 watts. Personally, I don't believe that with 10 extra grams of mass, the Power 32 gives 200 more watts of output... but all these numbers are fungible. Over time you'll get the hang of it. Vendors like EFLite will almost always recommend a specific motor or a range of specs for the motor of any ARF they sell. 

Oct 22, 2011, 09:26 AM  

Quote:
The design of the Pulse's cowl means that good cooling flow is achievable, just so long as you open up a large air outlet behind the wing. In this instance, 4W/g is not excessive. 

Oct 22, 2011, 11:11 AM  
Joined Jan 2009
7,643 Posts

Yep, on the right plane. And I've flown one or two that float nicely at 2025 watts per pound.
Quote:
All the numbers are fungible. 


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